Okay, via the photos I'd bet one side of the opening has all the stitching and the other side has none (reminding me "this little piggie went to market) I'd turn the none side under whatever amount you can spare, kind of crease it with your fingers. and then stitch by hand from the no stitch side into the STITCHING on the other side and back again. Go from the stitching to the folded side to sew up the seam.
THEN if you don't like it try something else.
Have you ever made something that requires you to leave one side unsewn? Something like a pillow, for example. This usually leaves something to be desired with the end result. I can always tell which side was the one I hand sewn closed. Annoying. Some wonderful person said machine stitch both sides of the pillow along what would be the seam lines on that last side. When you've finished everything and need to sew the last side closed fold the edges to the inside and stitch not the fabric but the extra row of stitching you made on both sides earlier.
Worked. You just sew into the stitches on one side and into the stitching on the other. Magic.
Probably will be the last time you ever have this problem!
Jois